The front to rear brake line has a lot of clamps that are in common with the fuel line, so you would probably want to do them at the same time. The two front brake lines can be done anytime, but I would do it before too much is installed in the engine compartment.
Do at the same time. For sure prior to engine assy. Brake lines in the engine bay are pain if you try to work around the engine. If the engine is in already - oh well have fun
I can tell you this much, DO NOT wait until the engine is in to put in your new brake lines. The one that goes from the m/c to the passenger front is a giant PITA to get in without scratching everything. Don't ask me how I know...